Hygrothermal performance of wooden beam heads in inside insulated walls considering air flows

Abstract

When planning an interior insulation for a building with existing wooden ceilings, the hygrothermal performance of the wooden beams’ heads inside the masonry need special consideration. Because of airflows due to thermal differences in pressure, warm and humid ambient air can penetrate the external construction components. Thus, condensation on the masonry surfaces resp. on the wooden beam will occur, which may lead to high moisture content and finally to processes of decay. Especially when thermally renovating a building using interior insulation the support area’s temperature level during winter is lowered significantly. Therefore, adequate airtightness of the inner shell is to be considered with such construction. At the same time, a certain degree of leakiness must be assumed in the planning stage since 100% airtight constructions are not achievable in practice.This paper will introduce a simulation model which takes into account convective hygrothermal processes in aerial spaces besides simulating temperature and moisture in porous materials. Using the example of a three-dimensional model of a wooden beam head, the influence of airflows will be shown. Damp permeation due to convection is going to be modeled by a condensation source at the wooden beam head’s end. The amount of condensate occurring there is determined by the mass flow from the inside into the construction as well as the temperature conditions on the surface. Furthermore, the size and set-up of the leakage between the interior air and the ceiling cavity is important. The results show that the durability of the wooden beams’ heads heavily depends on room air humidity and leakage size as well as the differences in pressure between the two aerial spaces. The results’ assessment is made by evaluating the risk for decay of the wooden components.